From what you describe, the fire started at your alternator. The clue is your battery needed a boost. My guess is the alternator was faulty rather than a bad battery. The alternator has diodes and such that can short or open and cause those parts within to overheat and catch fire. Even with 1 faulty diode, the alternator can still put out power/voltage, just not as much. (there are 3 windings and 1 diode would take out 1/3)